THIS invention relates to a method of manufacturing an object with microchannels provides therethrough, and more particularly, but not exclusively, to a method of manufacturing a micro heat exchanger with microchannels provided therethrough.
Microchannels are channels provided in various devices having any of the dimensions between 1 micron and 999 microns (Prakash and Kumar 2014), i.e. channels with a hydraulic diameter of less than 1 mm. Microchannels are primarily used in advanced biomedical, chemical, electronics, and mechanical engineering applications. Depending on the applications, microchannels have different shapes, sizes and structures and are fabricated from different substrate materials that exhibit properties preferable for a particular application.
For example, very fast and reliable compact computers require microprocessors with very high clock speed. However, the impediment of these advanced microprocessors is that they emit large heat flux densities which require novel cooling technologies in order to keep the temperature of the electronic components below critical levels (Zhao et al. 2002, Upadhya et al. 2006). At present, the heat flux densities emitted by microprocessors exceed the capability limit of the existing air cooling technologies that ultimately compromises the performance and reliability of the computers due to increased operation temperatures (Mihai 2011).
To address this problem, microfluidic cooling systems have been proposed as innovative thermal solutions for cooling the contemporary and next generation high speed computer microprocessors. Microfluidic cooling systems exhibit superior thermal extraction capabilities compared to other cooling technologies. In this regard, FIG. 1 presents the hierarchy of thermal extraction mechanisms performance, and shows how microfluidic cooling systems outperforms the other heat extraction systems.
A typical microfluidic cooling system is shown in FIG. 2 and comprises a micro heat sink, a micro pump, a micro condenser, and a fan. All these components operate on a closed loop micro-scale principle (Upadhya et al. 2006). During operation, the micro pump pumps the cooling liquid from the micro condenser to the micro heat sink, where the cooling liquid traverses the micro heat sink via the microchannels acting as heat transfer conduits. It is in these microchannels where the heat, which is conducted from the heat spreaders of the microprocessor, is transferred to the cooling liquid which in turn absorbs heat. The heated liquid then removes the heat to the micro condenser, where dissipation to the atmosphere occurs with the assistance of air cooling of the fan. The coordinated operation of these components accounts for the superior thermal performance of the microfluidic cooling systems (Upadhya et al. 2006).
Depending on the applications of microchannel-based devices, different types of materials are preferred. Polymeric and glass substrates are mostly used in biomedical and chemical devices, while silicon-based substrates and metallic substrates are used for electronics and mechanical engineering-related applications (Prakash and Kumar 2014). In recent years, polymeric substrate microfluidic devices started to exceed the use of silicon and glass substrates, mainly because of their low production costs and their high chemical resistance to an operating environment. Also, metallic microchannels have gained considerable attention as cooling devices in electronic and mechanical applications because many endothermic and exothermic reactions can be performed on such metallic substrates. The metallic microchannels can furthermore withstand corrosive environments, and can reach operating temperatures as high as 650° C. (Prakash and Kumar 2014).
A further important consideration is that shapes, sizes and structures of microchannels vary depending on the particular application for which it is to be used. Most common cross sections include rectangular microchannels, square microchannels, circular microchannels, half circular microchannels, U-shape microchannels and Gaussian beam shape microchannels (Prakash and Kumar 2014). Also, whilst most of the microfluidic channels have high area-to-volume aspect ratios, low area-to-volume aspect ratio channels are also not uncommon in applications such as particle separation devices (Prakash and Kumar 2014).
Studies have shown that different microchannels with different cross sections exhibits different heat removal performance, with the trapezoidal shape microchannels outperforming the other possible microchannels' shapes in terms of heat extraction capability (Asgari et al.). Also, this performance varies with the geometric dimensions of the microchannels (Gargi et al. 2013) which are also influenced by the technology used to fabricate those microchannels (Zhou et al. 2014). There is, however, a scarcity of micro heat sinks with trapezoidal channels due to the lack of robust microfabrication methods (Upadhya et al. 2006, Gargi et al. 2013).
The large scale fabrication of microchannels in typical substrates has always been a difficult task because of:                (1) the precision required in the manufacturing of such products; and        (2) the lack of suitable technologies to fabricate these devices (Prakash and Kumar 2014).        
The methods used for fabricating different types of microchannels include both conventional and nonconventional fabrication techniques. However, the contemporary microfabrication technologies for microchannels could be broadly categorized as additive or subtractive, depending on whether the material is added or subtracted during the microfabrication process, as summarized in FIG. 3. The main groups of these technologies are:                stereolithography,        chemical etching; and        micro-machining.        
The most common fabrication processes for microchannels are discussed in more detail below.
Micro-machining of microchannels is particularly suitable for the fabrication of individual personalized components rather than fabrication of large batch sizes. This group of methodologies evolved as a result of the advent of ultra-precision machining tools that can achieve high level of machining accuracy at high machining speeds, whilst also resulting in good surface finish on a large number of materials, such as steel, aluminum, brass, or plastics and polymers (Prakash and Kumar 2014). Micro-machining is the most diverse category of microfabrication technologies and is composed of advanced micro milling, laser cutting, and electrical discharge machining (EDM) or a combination of these processes. These processes do not require a very expensive setup, which enables them to be used to produce micro-devices in small quantity and at a reasonable cost (Prakash and Kumar 2014).
Advanced micro milling has a drawback of limited tool geometries, which makes it difficult to fabricate microchannels with sizes below 500 μm. Laser technology, which uses a collimated laser beam to groove substrates, is a time consuming process which is not suitable for mass production and the technology cannot fabricate preset geometric dimensions due to laser interaction with the materials. The EDM method has the challenge of low rate of material removal and therefore is also not well suited for mass production of micro heat sinks (Zhou et al., 2014). A common drawback of all the micro-machining processes is therefore that it is not suitable for the high volume mass production of microchannel devices.
Chemical etching, the most widely used subtractive technique for micromachining, could be described as pattern transfer by chemical or physical removal of material from a substrate, often in a pattern defined by a protective mask layer such as a resist or an oxide (Prakash and Kumar 2014). Chemical etching could be wet or dry. In dry etching, mostly utilized for glass and polymer base materials, the surface can be physically etched in the gas or vapour phase by ion bombardment, can be etched by a chemical reaction at the surface, or can be etched by combining the physical and chemical mechanisms. Wet etching is suitable for metallic substrates that react well with chemicals, but the process results in non-parallel walls on the glass surface and, as the channel etches deeper, the walls are also etched (Prakash and Kumar 2014). In addition, chemical etching method has very low productivity and the process does not lend itself to precise control of the geometric dimensions of the fabricated microchannels.
Lithography is one of the major fabrication techniques used to fabricate microchannels. This process enables the fabrication of many different types of topographies that are difficult to generate using other fabrication techniques (Prakash and Kumar 2014). The most widely used form of lithography is where pattern transfer from mask onto thin films is done by photolithography. In recent times, X-ray lithography has also been used to create polymer microchannels that, in contrast with ion-beam lithography and electron beam lithography, do not require the presence of vacuum and clean room facilities, which makes this process cheaper and faster (Prakash and Kumar 2014). Furthermore, LIGA—the German abbreviation for Lithography, Galvanoformung (electroplating) and Abformung (Molding)) enables the precise manufacturing of high aspect ratio microchannels ranging from 100 to 1000 microns, and enables the use of new building materials and the fabrication of a wider dynamic range of dimensions and shapes (Prakash and Kumar 2014). However, its applicability is restricted by high costs, as well as the production of toxic waste (Zhou et al. 2014).
The above limitations of contemporary microfabrication technologies provide overwhelming evidence that, up to now, there has been no robust method for the mass microfabrication of microchannels with trapezoidal cross-sectional profiles. Therefore, new methods for faster and cheaper production of these devices must be explored for sustainable development in this area, in particular since there is a growing demand for microchannels with trapezoidal cross-section for use in micro heat exchangers.
It is accordingly an object of the invention to provide a method of manufacturing an object with microchannels provides therethrough that will, at least partially, alleviate the above disadvantages.
It is also an object of the invention to provide a method of manufacturing an object with microchannels provides therethrough, which will be a useful alternative to existing methods.